"We talked to Magnussen, who was a very good candidate," the American told the Formula1.com website. "He was very well respected by the McLaren team.
"In fact if Romain had declined we would have taken Magnussen as the primary seat."
Magnussen, 23, raced for McLaren in 2014 and was reserve this season. He was told on his birthday last month that the team had no place for him.
News that makes sense
Your trusted source for staying up-to-date with the world around you. Get free daily news updates and analysis, straight to your inbox.
The Dane now looks sure to leave Formula One, with tailenders Manor the only team still to confirm drivers for 2016.
Ferrari-powered Haas, the first U.S. owned team since 1986, are making their debut next season with Grosjean and Mexican Esteban Gutierrez.
(Reporting by Alan Baldwin, editing by Greg Stutchbury)
